It is known that the density of a web roll riding on two parallel support rolls can be controlled during the winding operation by distributing the weight of the web roll among the support rolls. To this end, support rolls of the same diameter are located in different horizontal planes, or support rolls of different diameters are employed. It is further known that when the web roll is wound on the smaller-diameter roll, a more tightly wound, or denser, web roll is obtained than when it is wound on the larger-diameter support roll.
Even though these facts have been known for tens of years, to this day no fully satisfactory winding technique has been developed. In an attempt to overcome the drawback that the inner layers of the web roll are radially compressed and deformed in a starlike manner by the outer layers as the web builds up on the roll, provision has been made for a high roll density at the start of the winding operation. However, this approach entails an excessively high roll density in the outer layers of the web roll, which results in creases, ruptures and tears. To overcome these drawbacks, web rolls have been wound to a smaller diameter, with the density of the roll then being at a tolerable level even in the outer layers. Moreover, it has been found that in the unwinding of a web accuracy-related problems would arise in processing the web at the station located downstream of the roll. In the case of a sheet cutter, it has proved impossible to cut the sheet to the exact size required. It has been found that this is due to varying web tension. Measurements of the web-roll density over the entire diameter of the roll have shown that the density fluctuates greatly about a mean value.